(photo courtesy; Texas A&M U)
Appearance Potential Spectroscopy (APS) was developed in the 1970’s by R.L.Park and J.E Houston. This is one of the various spectroscopic techniques used for measuring the binding energies of core level electrons. It is also called X-Ray Photoelectron Spectroscopy is one of the modern technique for measuring the binding energy. APS is so called because one determines the incident energy at which the relaxation products of a certain core hole appears. In this method soft X-Ray excites the core electron and makes it to emit radiation whose energy spectrum measured. The short penetration depths of electrons used makes APS highly surface sensitive.
In short the experiment is done in vacuum and thermally excited electrons are used as a source of excitation. The voltage to the sample is increased in steps. Following the excitation of a core level electron, the system will de-excite and appear as an emission of Auger electrons or Soft X-rays. Depending on what is monitored, the appropriate name is given to that technique.
Principle
This technique is based on the principle of measuring the threshold energy for the creation of inner shell excited atom. Threshold energy provides Information about the binding energies of electrons in the core levels of surface atoms.
In Appearance Potential Spectroscopy the intensity of auger or X-Ray emission as a function of the excitation threshold (Ep) can be Determined. This technique also gives information about the unoccupied states of a sample. In this respect it can be termed as an inverse Auger electron Spectroscopy in which the density of occupied states are mapped. APS can be used as a tool for the investigation of electronic structure of solid surfaces.
In APS the energy of the exciting particle is recorded and not the energy of the decay of the excited states. But the spectrum will be simple and can be easily interpreted. Compared to other techniques the instrumentation is very simple making it a valuable tool for surface analysis.
Appearance Potential Spectroscopy can be classified into Soft X-Ray Appearance Potential Spectroscopy (SXAPS), Auger Electron Appearance Potential Spectroscopy (AEAPS) and Disappearance Potential Spectroscopy (DAPS).
Soft X-Ray Appearance Potential Spectroscopy (SXAPS)
Soft X-ray Appearance Potential Spectroscopy SXAPS is a member of the Appearance Potential Spectroscopy with very simple experimental apparatus. A filament is mounted near the sample and emits electrons which are accelerated towards the sample. X-rays generated within the sample are detected via photoelectrons generated by the X-ray within the detector. Due to the long mean free path of X-rays in matter, it might be thought that SXAPS is not particularly surface sensitive, but as it is a threshold technique, the incident electrons will only travel a short distance before they are unable to excite the level of interest. As X-rays are very inefficiently generated, SXAPS suffers from poor signal.
Auger Electron Appearance Potential Spectroscopy (AEAPS)
AES is a popular technique for determining the composition of the top few layers of a surface. It cannot detect hydrogen or helium, but is sensitive to all other elements, being most sensitive to the low atomic number elements. AES must be carried out in UHV conditions. A popular method of looking at buried layers with AES is to use the technique in combination with sputter cleaning.
DAPS -Disappearance Potential Spectroscopy
DAPS is used to detect ionization thresholds of core levels in atoms. This is done by bombarding the surface with electrons and modulating the energy of the primary beam. The electrons emitted from a surface can be detected with RFA and measuring the current to ground from one of the hemispherical grids. If the primary beam voltage is such that a core peak is on the edge of being excited, then changes in the current emitted from the surface can be detected using standard modulation techniques.
Applications
AEAPS is useful for studying Low Z materials and the signal is recorded in the second differential mode. Low temperature AEAPS can be use to conduct temperature variation studies.
APS technique is non-dispersive in nature. As signal is extracted from the background using potential modulation technique, its strength is proportional to the Unoccupied Density of States at the Fermi Level. In an APS spectrum, the peaks provide a means of elemental identification. The threshold can be used to measure the Binding Energy and the chemical shift correlates to changes in the chemical bonding.